The present invention relates to an image forming apparatus such as a copying machine, a facsimile, and a printer, each having therein a fixing device which employs a belt.
In prior art of fixing devices which are publicly known and generalized, employed are an endless belt maintained in a predetermined temperature range, or paired pressure applying rollers with the endless belt sandwiched in between, and therefore, a transfer material supporting thereon unfixed toner images faces the surface of the endless belt, and is conveyed and pressed onto the endless belt by the paired pressure applying rollers, and thereby, the toner particles making up images on the transfer material are heated and fixed.
However, in fixing devices which employ such an endless belt, the rollers are often deformed by heat, and the driving resistance factor of tension is very large in the nipping area (an area in which the transfer material is nipped by the belt and the pressure applying roller), and is also a very large factor on the belt supporting roller (which is a heating roller), therefore, a change of torque occurs on the belt driving roller which is the fixing roller, and also on the pressure applying roller which presses the transfer material onto the fixing roller, and then, slippage may occur between the paired pressure applying rollers and the belt, resulting in a slippage of the images on the transfer material and the belt.
In order to avoid the matter mentioned above, disclosed are several technologies, one of which is that the pressure applying roller and the heating roller are simultaneously driven by a driving source through gears (see patent document 1), and another technology is that the torque of either the separating roller or the fixing roller is controlled and when the torque is increased to a predetermined value, driving power is activated for the other roller, and as a result, the belt is tensioned not more than necessary, causing the belt to run stably (see patent document 2).
[Patent Document 1] Tokkai 2001-125422
[Patent Document 2] Tokkai 2002-251095